The present invention relates to a tungsten target that is used in forming gate electrodes, interconnections, or the like of an IC, an LSI, or the like by sputtering, as well as to a manufacturing method of such a tungsten target.
With recent increase in the integration density of VLSIs, studies are now being made to use materials having lower electric resistance values for electrodes and interconnection. In these circumstances, high-purity tungsten that has a small electric resistance value and is stable thermally and chemically is considered promising as a material for electrodes and interconnections.
In general, electrodes and interconnections of VLSIs are formed by the sputtering or CVD. And the sputtering is used more widely than CVD because sputtering systems are relatively simple in configuration and manipulation and the sputtering allows easy formation of a film at a low cost.
However, in forming electrodes or interconnections of a VLSI by sputtering, if defects called particles are formed on a resulting film surface, they may cause a failure such as a wiring defect. This is a factor of lowering the yield. To decrease the probability of occurrence of particles on a film surface formed, a tungsten target having a high density and fine crystal grains is required.
Among the conventional methods for manufacturing a tungsten target are a method disclosed in Japanese Patent Laid-Open No. 61-107728 in which an ingot is produced by an electron beam melting method and then hot-rolled, a method disclosed in Japanese Patent Laid-Open No. 3-150356 in which a tungsten powder is pressure-sintered and then rolled, and what is called a CVD-W method disclosed in Japanese Patent Laid-Open No. 6-158300 in which a tungsten layer is laid on one surface of a tungsten bottom plate by CVD.
However, the above-mentioned two methods have problems that a manufactured tungsten target is fragile because large crystal grains tend to be formed and that particle defects tend to occur on a film formed by sputtering.
On the other hand, although a tungsten target manufactured by the CVD-W method exhibits good sputtering characteristics, it has problems that manufacture of a target takes very long time and is costly. That is, the CVD-W method is less economical.
The object of the present invention is to provide a powder sintering method having a relatively simple manufacturing process, and to provide an improved method for manufacturing a tungsten target by improving the sintered characteristics of a tungsten powder used. The improved method should manufacture, stably at a low cost, a tungsten target having a high density and fine crystal structure that cannot be attained by conventional pressure sintering methods, and should suppress occurrence of particle defects on a film produced by using such a target.
It is known in the powder metallurgy that usually the degree of sintering increases as the grain size of a powder used becomes smaller, that is, the specific surface increases. However, even fine ones of high-purity tungsten powders on the market have as small specific surfaces as about 0.2 m2/g.
The inventors have studied a pressure sintering method using a tungsten powder having a large specific surface, and have succeeded in making the relative density 99% or more, average grain size 100 xcexcm or less and the oxygen content 20 ppm or less, by sintering a tungsten powder having a powder specific surface of 0.4 m2/g or more (BET method), preferably, 0.6 m2/g or more (BET method). As a result, a tungsten target for sputtering has been realized that can suppress occurrence of particle defects on a sputtered film.
The invention provides a manufacturing method of a tungsten target for sputtering, comprising the steps of hot-press-sintering a tungsten powder having a powder specific surface of 0.4 m2/g or more, preferably 0.6 m2/g, as measured according to a BET method in vacuum or a reducing atmosphere; and subjecting a resulting sintered body to hot isotropic (isostatic) pressure sintering (HIP).
In this manufacturing method, a tungsten target for sputtering that has a high density and a fine crystal structure can be manufactured more effectively by each of the following:
Producing a sintered body having a relative density of 93% or more by the hot press sintering.
Performing the hot press sintering at a temperature of 1,600xc2x0 C. or more and a pressure of 200 kg/cm2 or more.
Performing the hot isotropic pressure sintering (HIP) without capsuling.
Performing the hot isotropic pressure sintering at a temperature of 1,700xc2x0 C. or more and a pressure of 1,000 kg/cm2 or more.
The above methods make it possible to manufacture, stably at a low cost, a tungsten target for sputtering that has, after the hot isotropic pressure sintering, a relative density of 99% or more, an average grain size of 100 xcexcm or less, and an oxygen content of 20 ppm or less.
Usually, high-purity tungsten powders on the market having purity of 5N or higher have as small specific surfaces as 0.2 m2/g or less. To manufacture a tungsten target having a relative density of 99% or more by pressure sintering by using such a tungsten powder, a sintering temperature of 2,000xc2x0 C. or more is necessary. However, at such a high sintering temperature, the crystal grain size becomes as large as more than 100 xcexcm.
Pressure sintering at such a high temperature causes serious problems such as reaction between dies and tungsten (hot press) and reaction between a capsule material and tungsten (HIP). This results in cost increase.
It is known in the powder metallurgy that usually the degree of sintering increases as the grain size of a powder used becomes smaller, that is, the specific surface increases.
However, as mentioned above, even fine ones of high-purity tungsten powders on the market have as small specific surfaces as about 0.2 m2/g.
In view of the above, the inventors produced a tungsten powder whose specific surface is in a range of 0.4-1.4 m2/g by using ammonium methatungstate as a starting material and properly controlling the supply amount of a hydrogen gas and increasing the rate of eliminating a reaction product gas in hydrogen-reducing a tungstate crystal having purity of 5N or higher obtained by refining (purifying) the starting material.
The tungsten powder having such a large specific surface was hot pressed at a temperature of 1,600xc2x0 C. or more and a pressure of 200 kg/cm2 or more, whereby the relative density became 93% or more. Since closed pores were formed, HIP was possible without capsuling.
Although setting the hot press temperature at 2,000xc2x0 C. or more is effective for increasing the density, it causes such problems as increase in crystal grain size and reaction with dies. Therefore, it is preferable that the hot press is performed at 1,900xc2x0 C. or less.
Then, HIP was performed at a temperature of 1,700xc2x0 C. or more and a pressure of 1,000 kg/cm2 or more, whereby a tungsten target having a relative density of 99% or more and an average crystal grain size of 100 xcexcm or less was produced.
As the specific surface of a tungsten powder increases, the density of a tungsten sintered body increases and its crystal structure becomes finer, which facilitates density increase in the HIP and increases a post-HIP density.
The number of particle defects on films formed by using a tungsten target that was manufactured in the above manner decreased to a large extent.